Abstract: Our archival data indicates that 27% of older adults hospitalized with major depression and suicidality continue to report suicidal ideation (SI) three months after discharge. Having persistent post- discharge SI places individuals at very high risk for suicidal behaviors and death. Cognitive and affective mechanisms likely determine the course of post-discharge SI, but interrupting these processes remains challenging. This is, in part, because the upstream modifiable risk factors that exacerbate problems with cognition, affect, and SI in the post-discharge period are poorly understood. We propose that sleep-wake disturbances are potentially important contributors to the post-discharge prognosis. Sleep-wake disturbances plausibly influence the cognitive and affective mechanisms that underlie SI. But many potentially relevant sleep-wake factors have been identified, and there is not yet evidence regarding which mark or drive the mechanisms that perpetuate SI after discharge. To begin filling these gaps, we propose a prospective observational pilot study examining sleep-wake disturbances, alongside other putative suicide risk factors, in the critical post-discharge period. This Exploratory/Developmental Research (R21) proposal brings together experts in sleep-wake rhythms and late-life depression (Smagula, PhD, PI), sleep medicine (Buysse, MD), late- life suicide (Szanto, MD, Co-I), cognitive aging (Butters, PhD), and time series analytics (Krafty, PhD, Co-I). We will monitor the course of suicidality over 12-weeks in 70 adults (age 55-75, with non-psychotic major depressive disorder, current active SI, and a recent psychiatric hospital discharge). Over 6 weeks, we will perform high-resolution data collection including: assessing 24-hour sleep-wake patterns (using actigraphy and diary); measuring daily suicidality levels (passive ideation, active ideation, and planning) and affect (depression and anxiety); and administering weekly home-based tests of cognitive functions previously linked with sleep and suicide (i.e., attention, response inhibition, and reversal learning performance). Our testable hypotheses based on existing evidence are that: (1) weekly measures of short sleep duration and sleep-wake rhythm disruption will temporally precede and independently predict a worse course of post-discharge SI; and (2) these sleep-wake factors will relate to the course of SI, in part, via their adverse effects on cognition and affect. Given their important roles, we will also evaluate the effects of psychosocial factors and assess their relationships with sleep. Collecting these novel data will enable analyses: (1) ranking the effect sizes of putative risk factors including sleep-wake, psychosocial, affective, and cognitive measures; (2) examining the temporal relationships between risk factors; and (3) preliminarily testing mediational models. Results from this study will inform the development of confirmatory studies ultimately leading to novel, evidence-based, interventions. If sleep-wake factors have a role in determining post-discharge suicide risk, the potential for clinical translation is high, given that sleep-wake risks could be modified and/or monitored after discharge.